(a) Field of the Invention
The present invention relates to a method of synchronization in a distributed wireless communication system and a terminal supporting the same.
(b) Description of the Related Art
Wireless communication systems may be roughly classified into synchronous communication systems and asynchronous communication systems.
In a synchronous wireless communication system, terminals are synchronized to a common reference clock, and the time domain is divided into repeated intervals called frames. Each frame is subdivided into smaller intervals called slots. A system where a signal is transmitted or received within a slot is referred to as a synchronous wireless communication system.
Synchronous communication may be classified into centralized synchronization communication and distributed synchronization communication. A typical example of centralized synchronization communication is a cellular system, where a base station provides synchronization signals. A terminal transmits and receives signals by using radio resources within a frame structure defined with respect to a synchronization signal (timing reference signal) provided by the base station. Distributed synchronization communication is a method of synchronization in which terminals within a network participate in the transmission of synchronization signals, rather than in which a specific device transmits synchronization signals. Distributed synchronization communication allows for synchronization between terminals within a network, even without a communication infra-structure or a specific device that transmits synchronization signals. Hence, this method is suitable for device-to-device (D2D) communication networks or temporary networks (ad hoc networks).
An asynchronous wireless communication system is a system in which terminals transmit and receive signals without the use of a fixed reference clock. In the asynchronous wireless communication system, the terminals constantly monitor radio channels for signal reception because they do not know when signals are to be received. Upon detecting a wireless packet while monitoring, the terminal estimates the start point of the packet using a preamble signal contained in the wireless packet, and then reads packet information. Asynchronous communication can be easily implemented because it uses no fixed reference clock, and is therefore used mainly for systems that do not require high resource efficiency. However, this type of communication consumes much electric power since terminals need to monitor radio channels all the time.
The following is a type of distributed synchronization communication. Each terminal cyclically transmits a synchronization signal by using its own time synchronization, i.e., reference clock. Then, each terminal receives synchronization signals transmitted by neighboring terminals, and then synchronizes its clock by using the received synchronization signals.
The synchronization signals used herein may include a pulse signal, a ZC (Zadoff-Chu) sequence, an m-sequence, a chirp signal, etc. The terminals cannot receive the synchronization signals transmitted by neighboring terminals during synchronization signal transmission because synchronization signal transmission and reception cannot occur simultaneously. In this method, however, if terminals are densely populated, synchronization signals transmitted by the terminals may be received while overlapping each other, thus causing an error in the estimation of the time of receipt of the synchronization signals. Moreover, if the same radio channel is used by neighboring heterogeneous networks in an unlicensed frequency band, synchronization signal transmission may not occur or a transmitted synchronization signal may be lost due to collision.